The present invention relates to polygon optical scanning systems. The present invention has particular applicability in laser printing, printed circuit board inspection, wafer inspection, reticle pattern inspection, and photolithography.
In conventional polygon scanning systems, a beam of light, such as from a laser light source or a lamp, is directed to illuminate a spot on a reflective facet of a rotating polygon, which typically has many such reflective facets. The reflected light beam from the facet is typically passed through an optical system for magnification and/or focusing, and then impinges on the surface of an object, such as a semiconductor wafer coated with photoresist, or the printing drum of a laser printer. As the polygon rotates, the reflected light beam is scanned across the surface of the object.
The data rate that can be achieved with a conventional polygon scanning system is dependent upon, inter alia, the rotational velocity of the polygon, the number of facets of the polygon, and the xe2x80x9cduty cyclexe2x80x9d of the polygon. The duty cycle can be defined as the portion of the area of each facet that is used for scanning. The duty cycle is typically less than xe2x80x9c1xe2x80x9d for conventional systems, because scanning cannot be performed when the incident light beam is transitioning from illuminating a first facet to illuminating a second, adjacent facet, as a result of the rotation of the polygon. During such transistions, a portion of the beam is deflected in one direction (from the first facet) and another portion of the beam is deflected in another direction (from the second facet), thereby preventing scanning of the substrate surface by the beam.
Another factor affecting the data rate of polygon scanning systems is the number of reservable spots (also called xe2x80x9cpixelsxe2x80x9d) along a scan line, which can be expressed as the xe2x80x9cscan anglexe2x80x9d xcex8sl, or total angle that the facet reflects the light beam, divided by the angle of diffraction xcex8d due to the finite diameter of the light beam:
number of spots on scan line=xcex8sl/xcex8d, wherexe2x80x83xe2x80x83(1) 
xcex8sl=4Π/number of facets, andxe2x80x83xe2x80x83(2) 
xcex8d=wavelength of light beam/beam diameter.xe2x80x83xe2x80x83(3) 
As the foregoing equations 1-3 suggest, two contradictory conditions exist in prior art polygon scanning systems. To enlarge the number of spots on a line for improved system performance, the diameter of the beam needs to be increased (thereby decreasing diffraction). However, increasing the beam diameter decreases the duty cycle, because more facet area overlaps when the beam is transitioning from one facet to another, thereby reducing efficiency. This inherent tradeoff limits the pixel rate that can be achieved in conventional systems.
Prior art attempts to increase pixel rate have had limited success, and have related to illumination methodology, such as modifying the beam incident angle, and the shape and/or number of facets. For example, the number of facets can be increased, but the result is a decreased duty cycle. Alternatively, beam diameter can be increased such that the spot covers the entire facet (known as xe2x80x9coverfillxe2x80x9d), but if a laser beam is made large enough to overfill a facet and have the proper intensity and uniformity, it typically has low optical efficiency, wasting a significant amount of energy. Still further, the length of the facets can be enlarged, resulting in a smaller number of facets, each facet scanning a larger angular range. However, this increase in xcex8sl is of limited utility, since light reflected from the facets is typically directed through an optical system (i.e., lenses), and such systems do not have a large angular range, for example, about 10 degrees to 20 degrees.
There exists a need for a polygon scanning system having an increased duty cycle and data rate, thereby increasing efficiency and system throughput.
An aspect of the present invention is a polygon scanning system which maximizes data rate while increasing its duty cycle, thereby significantly improving the efficiency of the system.
According to the present invention, the foregoing and other aspects are achieved in part by a polygon scanning system comprising a polygon having a reflective facet, a rotation mechanism for rotating the polygon, and a light source for directing a plurality of light beams to impinge on the facet such that each light beam impinges on the facet at a different incident angle. Each light beam is reflected by the facet to scan a particular portion of a surface of a substrate during a respective time interval when the rotation mechanism is rotating the polygon. Each of the plurality of light beams is reflected onto the substrate surface using a respective portion of the facet surface, such that the sum of the respective portions of the facet surface used to reflect the light beams is a very large percentage of the total surface area. Thus, the system has a duty cycle of close to 100 percent as well as a high data rate.
Another aspect of the present invention is a method comprising rotating a polygon having a reflective facet; directing a first light beam to impinge on the facet at a first incident angle such that a first light beam is reflected by the facet to scan a first portion of a surface of a substrate during a first time interval while the polygon is rotating; and directing a second light beam to impinge on the facet at a second incident angle such that a second light beam is reflected by the facet to scan a second portion of the surface of the substrate during a second time interval subsequent to the first time interval while the polygon is rotating.
Additional aspects of the present invention will become readily apparent to those skilled in this art from the following detailed description and appended claims, wherein only the preferred embodiments of the present invention are shown and described, simply by way of illustration of the best mode contemplated for carrying out the present invention. As will be realized, the present invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as restrictive.